Touch sensing systems (“touch systems”) are in widespread use in a variety of applications. Typically, the touch systems are actuated by a touching object such as a finger or stylus, either in direct contact or through proximity (i.e. without contact) with a touch surface. Touch systems are for example used as touch pads of laptop computers, in control panels, and as overlays to displays on e.g. hand held devices, such as mobile telephones. A touch system that is overlaid on or integrated in a display is also denoted a “touch screen”. Many other applications are known in the art.
To an increasing extent, touch systems are designed to be able to detect two or more touches simultaneously, this capability often being referred to as “multi-touch”. There are numerous known techniques for providing multi-touch sensitivity, e.g. by using cameras to capture light scattered off the point(s) of touch on a panel, or by incorporating resistive wire grids, capacitive sensors, strain gauges, etc into a panel.
US2004/0252091 discloses an alternative technique which is based on frustrated total internal reflection (FTIR). Light sheets are coupled into a panel to propagate inside the panel by total internal reflection. When an object comes into contact with a surface of the panel, two or more light sheets will be locally attenuated at the point of touch. Arrays of light sensors are located around the perimeter of the panel to detect the received light for each light sheet. A coarse reconstruction of the light field across the panel surface is then created by geometrically back-tracing and triangulating all attenuations observed in the received light. This is stated to result in data regarding the position and size of each contact area.
US2009/0153519 discloses a panel capable of conducting signals. A “tomograph” is positioned adjacent to the panel with signal flow ports arrayed around the border of the panel at discrete locations. Signals measured at the signal flow ports are arranged in a sinogram (b) and tomographically processed to generate a two-dimensional representation (x) of the conductivity on the panel, whereby touching objects on the panel surface can be detected. The presented technique for tomographic reconstruction is based on a linear model of the tomographic system, Ax=b. The system matrix A is calculated at factory, and its pseudo inverse A−1 is calculated using Truncated SVD algorithms and operated on a sinogram b of measured signals to yield the two-dimensional (2D) representation of the conductivity: x=A−1b. The suggested method is both demanding in the term of processing and lacks suppression of high frequency components, possibly leading to much noise in the 2D representation. US2009/0153519 also makes a general reference to Computer Tomography (CT). CT methods are well-known imaging methods which have been developed for medical purposes. CT methods employ digital geometry processing to reconstruct an image of the inside of an object based on a large series of projection measurements through the object.
WO2011/139213 discloses an improved technique for tomographic reconstruction based on signals from a touch system that operates by transmission of signals across a touch surface. The signals, which represent detected energy on a plurality of actual detection lines across the touch surface, are processed to generate a set of matched samples, which are indicative of estimated detected energy for fictitious detection lines that have a location on the touch surface that matches a standard geometry for tomographic reconstruction. This technique enables the touch system to be designed with any arrangement of actual detection lines across the touch surface, while still allowing for the use of conventional tomographic reconstruction algorithms, which generate an interaction pattern that represents the location of objects on the touch surface.
As will be described with reference to FIGS. 6A-6B in the detailed description of the present application, the Applicant has identified a need to improve the spatial resolution of the interaction pattern that is obtained when the teachings of WO2011/139213 are applied to generate fictitious detection lines that are matched to a parallel geometry on the touch surface. An improved spatial resolution may be achieved by increasing the number of actual detection lines for a given size of the touch surface. However, this comes with added cost and complexity since the number of emitters and sensors needs to be increased. Furthermore, increasing the number of detection lines will increase, significantly, the number of computations in the tomographic reconstruction processing. In touch systems, the available time for generating the interaction pattern and identifying the touches is limited, since the touch detection generally is performed in real time. At the same time, the touch system may be restricted in terms of processing speed or storage capacity, e.g. due to constraints imposed by a desire to reduce costs, limit power consumption, provide a certain form factor, etc. There is thus a need to improve the spatial resolution for a given number of detection lines.